Supporters of Egypt's ousted President Mohammed Morsi carry an injured man to a field hospital following clashes with security forces at Nasr City on Saturday. / Hassan Ammar, AP

by Sarah Lynch and Oren Dorell, Special for USA TODAY

by Sarah Lynch and Oren Dorell, Special for USA TODAY

CAIRO - Secretary of State John Kerry expressed "grave concern" after dozens were killed in violent clashes between security forces and supporters of former president Mohamed Morsi on Saturday morning in one of the deadliest spells of unrest since the 2011 uprising against Hosni Mubarak.

Kerry on Saturday called on Egyptian leaders to release detained political leaders so that a meaningful political dialogue can proceed.

"This is a pivotal moment for Egypt," Kerry said after consulting with European Union foreign policy chief Catherine Ashton and speaking to Egypt's interim vice president and foreign minister. "The final verdict" on the revolution that began more than two years ago "is not yet decided," Kerry said. "But it will be forever impacted by what happens right now."

The Egyptian Health Ministry said at least 72 people were killed in Saturday's violence. More than 200 were wounded in Cairo on Saturday, the Health Ministry added.

The violence began overnight when supporters of Morsi tried to extend a sit-in to a major road and police fired tear gas, according to reports. Thousands of Morsi supporters have held a sit-in for more than three weeks outside Rabaa Al Adawiya Mosque in Nasr City. They demand that Morsi be reinstated as president and that dozens of Muslim Brotherhood leaders who were detained following his July 3 ouster be released.

Egyptian prosecutors announced last week they are investigating Morsi for his involvement in planning a deadly 2011 prison break that the government says was perpetrated by Hamas, the State-Department Palestinian terrorist group that controls the Gaza Strip.

The conflict turned deadly, and bodies covered in bloody white cloths lined the floor of a makeshift hospital Saturday morning. Many suffered birdshot and gunshot wounds in the head and upper body, according to reports.

In his afternoon statement, Kerry said Egyptian authorities are obligated to respect Egyptians' right of peaceful assembly and expression, including the ongoing sit-in demonstrations. He called for an independent and impartial inquiry of the killings and - in an apparent reference to Brotherhood leaders who support Morsi, the political opposition that called for Morsi's ouster, and the military - he called on "all of Egypt's leaders across the political spectrum to act immediately to help their country take a step back from the brink."

A meaningful political dialogue, as called for by interim government officials, requires participants who represent all political elements of Egyptian society, Kerry said. So the USA "reiterates our call for an end to politicized detentions and the release of political leaders consistent with the law."

Differing narratives emerged over who was to blame for the escalation.

In a televised news conference Saturday afternoon, Interior Minister Mohamed Ibrahim denied that police used live ammunition and accused the Muslim Brotherhood of exaggerating the number of people killed. He said security forces plan to deal with remaining protesters "soon" without offering a precise timeline.

At least one video, however, that appeared to be taken Saturday morning showed security forces firing on demonstrators. It also showed young men throwing rocks, presumably in the direction of security forces.

A ministry of the interior statement said 14 police officers were injured, including two with gunshot wounds to the head.

The Brotherhood said its sit-in was attacked by police, snipers and thugs and called the event a massacre.

Waiel Yahya, a member of the Brotherhood's Freedom and Justice Party who was at the scene of the clashes, said that the field hospital in Nasr City had to stop receiving patients who came in with wounds, some of which were critical.

"Now the hospital cannot hold anyone because there is no medicine and not enough doctors, not enough supplies," said Yahya, who heads the media center at Rabaa Al Adawiya. "Anyone who comes is now directed to a hospitals nearby â?¦ There is no place to put the injured."

Yahya said protesters in Nasr City will continue the sit-in, noting that security forces "stopped firing" by late Saturday morning.

The army chief unseated Morsi, the nation's first freely elected leader, earlier this month and ushered in a transition plan for fresh parliamentary and presidential elections.

Morsi has been held by authorities since then, and on Saturday, Ibrahim said he may be transferred to the same prison where Mubarak is being held. Morsi has been accused of a slew of crimes, including conspiring with the Palestinian group Hamas to commit violence in Egypt.

Almost three weeks ago, more than 50 people were killed when security forces opened fire on pro-Morsi demonstrators outside a Republican Guard facility, where many believed Morsi was being held.

Saturday's renewed violence followed a day of protests by opposing political camps. Al-Sisi had called for Egyptians to take to the streets to give him and security forces a mandate to "confront possible violence and terrorism."

Egyptians supporting the military heeded the call. They converged on Tahrir Square and outside the presidential palace Friday afternoon, cheering in support of the military and holding pictures of Al-Sisi.

"No to terrorism," said Amin Ahmed, 43, standing in Tahrir Square, where a festive rally continued Saturday morning. "We are here for Egypt."

Nearby, pro-Morsi demonstrators seethed with anger. Many marched toward Rabaa Al Adawiya Mosque to insist that Morsi is the nation's legal president and demand his reinstatement.

"We came here to defend our votes in elections," said Hamed Al Mekabty, 31, from the Egyptian city Mansoura. "We are all against the military coup and came to say to Sisi: 'Go away, we don't want you.' "

At least eight people were killed in clashes that erupted Friday between Morsi supporters and opponents in Alexandria, the state news agency said. Over the past several weeks, dozens more have been killed in clashes, and attacks on security outposts have spiked in the Sinai Peninsula.

As the nation remains divided and inflamed, Saturday's violence further complicates the situation.

"The more casualties we witness every day, the more difficult it will be to achieve reconciliation between the two camps," said political analyst Mazen Hassan in Cairo. "And it will only make it more difficult for a dialogue to take place between all factions."